The present invention concerns a profiling system for determining optimal process settings in a conveyor oven, such as a reflow oven, used for reflowing solder paste on printed circuit boards.
Conveyor ovens (also called furnaces) are used in a variety of industries including the electronics, baking, and painting industries. Generally, conveyor ovens have multiple heating zones and one or more cooling zones through which parts are conveyed. The heating zones are thermally isolated from each other by air curtains or other means. Such thermal isolation allows each zone to be maintained at a temperature that differs from other zones in the oven. A particular advantage of conveyor ovens with multiple heating zones is that products can be heated to different temperatures at different times as they pass through the oven.
In the electronics industry, conveyor ovens known as reflow ovens are used to mount integrated circuits and other electronic components to printed circuit boards (PCBs) with solder paste. Typically, the soldering process within a conveyor oven can be characterized by the following phases: the preheat or ramp phase, the dwell or soak phase, the reflow or spike phase and the cooling phase. In the preheat phase, the solder paste is heated from room temperature to a preheat temperature to promote evaporation of the solvents, or carriers, in the solder paste. During the soak phase, the solder paste is permitted to xe2x80x9csoakxe2x80x9d for a predetermined period of time at a temperature range at which the flux in the solder paste becomes active. In the reflow phase, the solder paste is heated above the liquidous, or melting temperature of the solder for a predetermined period of time sufficient to permit reflow (i.e., wetting) of the solder paste. In the cooling phase, the solder joint solidifies, thereby electrically bonding the components to the circuit board.
Typically, the thermal requirements for a solder paste (also called solder paste specifications) for the preheat, soak and reflow phases are specified by the manufacturer of the paste. Generally speaking, xe2x80x9cprofilingxe2x80x9d is the process of determining the process settings for a part that will best satisfy the thermal requirements of the solder paste. Such process settings may include, for example, the temperature settings of each oven zone and the oven conveyor speed.
Devices for measuring the temperature profile of a part conveyed through an oven (i.e., the temperature response of the part) are known. For example, electronic data loggers (also called data collectors or monitors) have been developed that attach to a test PCB. One such data logger, the SuperM.O.L.E.(copyright) Gold temperature profiler, is an oven profiler sold commercially by Electronic Controls Design, Inc., of Milwaukie, Oreg. Beyond the M.O.L.E., the test PCB has various thermocouples strategically placed thereon. Traditionally, each thermocouple is connected directly to the electronic data logger. The electronic data logger is physically spaced apart from the PCB so as not to affect the heating of the PCB and thereby cause inaccurate temperature profiling. The data logger stores temperature information measured by the thermocouples, which information can be processed to determine and control the optimal temperature profile of the oven.
Typically, in the past, profiling was accomplished using a xe2x80x9ctrial and errorxe2x80x9d technique in which an operator, prior to a production run, guesses or estimates initial settings for the oven controls. Usually, the operator starts with a xe2x80x9cknown temperature profile,xe2x80x9d that is, the oven settings for which the resulting temperature response of a similar PCB conveyed through the same oven is known. An instrumented test assembly (e.g. a test assembly coupled to a data logger) is conveyed through the oven at the initial settings. The data logger is then removed from the test assembly and placed in a reader to obtain the stored data, which represents a temperature response of the assembly. The temperature response of the assembly is compared to the thermal requirements of the solder paste and other components on the board. The operator guesses or estimates changes to the oven settings for reducing the difference between the temperature response of the assembly and the thermal requirements of the process to within an allowable tolerance. The operator implements the guess by manually adjusting the oven settings and repeats the process over and over until the appropriate thermal requirements for the solder paste are reached. Of course, this process is disadvantageous in that it results in a significant number of test boards being scrapped, requires a relatively high level of operator skill and is time consuming. Other industries using conveyor ovens have experienced similar problems in setting ovens for a production run.
A need therefore exists for an improved system for profiling a conveyor oven.
A method according to one embodiment is used for controlling a reflow soldering process in a multiple zone conveyor oven. An ideal temperature profile is obtained from the solder paste specifications of the solder paste to be reflowed in the oven. The ideal temperature profile of the paste may include a preheat phase, a soak phase, a reflow phase and a cooling phase. The ideal profile is applied to the oven by aligning the beginning of each phase of the ideal profile with the forward end of an oven zone so as to form a target profile that fits the oven. Once a target profile is obtained, the set points of the oven zones are determined for effecting the target profile when the solder paste is conveyed through the oven.
In a specific approach, one or more profiles may be obtained by aligning the beginning of each phase of the ideal profile with a forward end of an oven zone. The profile that best fits the oven is selected as a target profile from which the set points of the oven are determined.
After setting the set points of the oven, the temperature profile of the solder paste is measured as it is conveyed through the oven. The temperature profile of the solder paste is compared to the target profile. If there is any deviation between the temperature profile of the solder paste and the target profile, the set points of the oven are automatically adjusted to reduce any such deviation.
According to another embodiment, an apparatus is provided for controlling a soldering process in which solder paste is reflowed in a multiple zone conveyor oven. The apparatus includes means for providing an ideal temperature profile derived from the solder paste specifications of the solder paste to be reflowed in the oven and means for generating a target temperature profile for the soldering process in which the beginning of each phase coincides with the forward end of an oven zone.
The apparatus may include a database containing solder paste specifications for a plurality of solder pastes and means for selecting a solder paste from the database to be reflowed in the oven. The apparatus also may include means for displaying the target temperature profile and means for adjusting the temperature of the target profile at a point where the target profile coincides with the forward end of an oven zone.
The foregoing and other objects, features, and advantages of the invention will become more apparent from the following detailed description of several embodiments, which proceed with reference to the accompanying figures.